(n.) Geophysics. A type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal. See: Surface Wave
Schlumberger Ltd. (The World's Leading Supplier Of Technology, Integrated Project Management And Information Solutions to Customers Working in The Oil and Gas Industry Worldwide), Schlumberger Oilfield Glossary Cedex, France 2014. http://www.glossary.oilfield.slb.com/en/Terms/sym/ a.aspx, (Ergo - Brit) {766}